In recent years, in order to improve performance of various safety devices, for example, of a seat belt and an airbag provided on a vehicle, operations of such safety devices may be controlled in accordance with a weight of an occupant seated on a seat of the vehicle. For example, in a case where the occupant seated on the seat does not wear the seat belt, a warning is generally displayed for indicating that the seat belt is not fastened after the seating of the occupant is detected. In addition, according to regulations in the North America, an airbag is required to be deployed at a vehicle crash in a state where an adult is seated on a passenger seat. In a case where a child safety seat is installed on the passenger seat to face the back of the vehicle so that an infant or a child faces a driver of the vehicle, an airbag deployment is restricted because an impact of the air bag deployment has an adverse effect on the infant or the child. An adult occupant is determined with reference to a weight of a small body size adult female. A separate reference is defined for determining an infant or a child. Accordingly, correctly and appropriately determining the weight of the occupant is extremely important for a safety aspect.
JPH09-207638A, hereinafter referred to as Reference 1, discloses an occupancy detection apparatus that determines whether or not an occupant is seated on a seat by detecting a load applied on the seat. The occupancy detection apparatus is provided with two load sensors at a multiple number of seat attachment portions for obtaining a load sum value of the two load sensors to determine whether or not an occupant is seated on the seat. Normally, four seat attachment portions are provided, however, the occupancy detection apparatus disclosed in Reference 1 requires minimum two locations of the seat attachment portions so that an occupancy detection apparatus having a simple configuration as a whole and advantageous in reducing cost may be provided.
JP3991740B, hereinafter referred to as Reference 2, discloses an occupancy detection apparatus for determining whether an occupant seated on a seat is an adult or a child. The occupancy detection apparatus disclosed in Reference 2 includes first load sensors and second load sensors for detecting a load applied on the seat at a portion of the seat near a buckle of a seat belt device and a portion of the seat in the opposite direction to where the buckle is provided. The occupancy detection apparatus also includes a detection means for detecting that a tongue plate is inserted into the buckle and a determination means that determines the occupant as an adult in a case where a total load calculated from values detected by the first load sensors and the second load sensors equals to or more than a predetermined threshold value. In addition, the determination means determines that the occupant is an infant or a child even on a condition where the total load equals to or more than the threshold value in a case where a history of a difference between the values detected by the first load sensors and the second load sensors indicates an increase to a value more than a predetermined value and furthermore a detected value by the first load sensors indicates an increase within a time range around a time point where the tongue plate has inserted. Accordingly, the occupancy detection apparatus disclosed in Reference 2 may prevent a false determination of the occupant as an adult in a case where the occupant seated on the seat is actually an infant or a child having a weight slightly lower than the threshold value and the seat belt of the infant or the child is fastened by a different passenger. Note that, according to Reference 2, two first load sensors and two second load sensors are provided on the four seat attachment portions in order to detect the total load.
The occupancy detection apparatus disclosed in Reference 1 is provided with a minimum number of load sensors in order to reduce cost and weight of the apparatus. Accordingly, the occupancy detection apparatus may determine whether or not an occupant is on a seat, however, may provide difficulty on distinguishing the occupant on the seat from an adult and a child safety seat. For example, the occupant on the seat is difficult to distinguish from an adult and a child safety seat in a case where the child safety seat is retained to the seat by engaging a tongue plate of a seat belt device with a buckle. A load sensor provided at a location near and beneath the tongue plate receives a large load applied from a person attempting to attach the child safety seat, the person who applies a part of a weight of the self on the child safety seat to engage the tongue plate with the buckle. As a result, the load sum value detected by the load sensors may become larger than the threshold value for determining the occupant as an adult, which may lead to a false determination of the child safety seat as an adult.
The occupancy detection apparatus disclosed in Reference 2 takes into account of a temporary increase of the load due to a seat belt attachment process so that the occupancy detection apparatus may distinguish the occupant on the seat from an adult and a child safety seat with high accuracy. Nevertheless, providing a load sensor to each of the four seat attachment portions results in increasing cost and weight of the apparatus.
A need thus exists for a seat occupancy determination apparatus, which is not susceptible to the drawback mentioned above